This invention, an improvement over my U.S. Pat. No. 4,862,788, relates to valve positioning devices used to insure the correct position of a valve stem and the like in relationship to electronic signals originating from a separate controlling instrument which is part of process control equipment. There are numerous state-of-the-art devices that are capable of performing similar functions, for example, U.S. Pat. No. 3,293,992 describes such a device being an integral part of a piston and cylinder mechanism.
Most state-of-the-art positioning devices are rather sophisticated and complex structures, wherein the valving means are usually of a two stage amplifying variety, which makes these devices sensitive to impurities in instrument air such as water or dust.
Secondly, any two stage servo-device tends to be dynamically unstable under certain conditions.
Another drawback of present state-of-the-art devices is the fact that the mechanical interconnection between the sensed valve stem position and the spring force generated on top of a signal diaphragm (to generate feedback) is done with levers whose tilting motion is converted into rotary motion via round shafts supported by guide bushings and through other rotary to linear conversion mechanisms. All of these tend to be complex and, in addition, subject such shaft bearings to atmospheric corrosion and dirt.
My invention overcomes these difficulties by providing for a positioning device that employs only a simple one stage three-way valve to yield superior dynamic stability and employs only one stamped feedback lever without need for a separate rotary to linear conversion mechanism but instead employs two opposed pivot points, used both as support and to provide guidance for the required tilting action. Additionally, my positioning device uses a simple set screw to selectively block the exhaust flow from said three-way valve, therefore providing a simple but effective means to change not only the sensitivity but also the speed of response of my positioning device. Additionally, my invention employs a simply attached current to air pressure converter which then converts a basically pneumatic positioning device to one operated from an electrical current. Finally, as is made apparent in my description of the invention, the device is very simple, it consists of very few parts and, therefore, can be manufactured at low cost.
These and other features and advantages will be better understood from the following detailed description.